This is not a pure refactoring, but almost. Right now we are in a weird
situation where we have two inconsistencies:
* `__set__()` is handled in `checker.py` while `__get__()` is handled in
`checkmember.py`
* rules for when to use binder are slightly different between
descriptors and settable properties.

This PR fixes these two things. As a nice bonus we should get free
support for unions in `__set__()`.

Fixes #18988

This should be a minimal change to restore backwards compatibility for
an edge case with forward references.

Fixes #18967
Fixes #16494
Fixes #15793
Fixes #12949

As you can see from updated test cases, it is kind of gray area, so
whether we go this way will depend on the `mypy_primer` results (and
also potentially on Dropbox internal code bases, where the above issue
may cause problems).

Fixes #18856. This should be done by `TypeAnalyzer.anal_array` but is
not - semanal only invokes its own wrapper around `anal_type`

---------

Co-authored-by: Ivan Levkivskyi <levkivskyi@gmail.com>

…1.16

The feature was introduced in #18727.

Also discuss `mypyc_attr(native_class=<...>)`.

Type objects as enum values are supported at runtime.

Fixes #19151.

)

This reverts commit 43ea203 (#17678).

The commit caused a regression (#19139). If we can't fix the regression
soon enough, reverting the original change temporarily will at least
unblock the mypy public release. The reverted PR can be merged again
once the regression is fixed.

Related to #18739.